Continuously variable transmission

ABSTRACT

The invention concerns a continuously variable belt-drive transmission, especially for motor vehicles. In the pressure less state of the transmission, the axially displaceable cone pulley of the input shaft is loaded with an axial force by a spring system in a manner such that the contact pressure increases on the belt-drive mechanisms and the ratio of the transmission is reduced.

[0001] According to the preamble of the main claim, the inventionconcerns a continuously variable belt-drive transmission, especially formotor vehicles.

[0002] Such belt-drive transmissions are generally known. Continuouslyvariable belt-drive transmissions usually comprise a variator for ratioadjustment which has a first cone pulley pair upon an input shaft and asecond cone pulley pair upon an output shaft, and a belt-drive meansrunning between the cone pulley pairs. Each cone pulley pair consists ofone first pulley (a fixed pulley) stationary in an axial direction andone second cone pulley (an adjusting pulley) displaceable in an axialdirection. The input shaft of the variator is usually designated as theprimary shaft and the first cone pulley pair accordingly as the primarypulley pair. The output shaft of the variator is, as a rule, similarlydesignated as the secondary shaft and the second cone pulley pair as thesecondary pulley pair. The primary pulleys and secondary pulleys andthus the ratio are axially adjusted by a pressure medium. The pressuremedium is usually conveyed via ducts onto one or more pressure chambersof the primary or secondary pulley, and a pressure-medium pump makingavailable the oil pressure needed.

[0003] The pressure supply of the whole transmission is mostly madepossible via a pressure-medium pump directly coupled with the inputrotational speed. The volume of oil conveyed depends therefore directlyon the input rotational speed. When now the input rotational speed dropsdown to zero, the transmission is no longer supplied with oil and aso-called pressureless state appears. In this state, the maximum ratiois usually adjusted. At the same time, the primary pulleys stay insofaras possible, apart from each other and the belt-drive means rests on theprimary side upon the smallest possible friction radius. The frictionradius of the belt-drive means upon the output shaft, which radius ispreset by the secondary pulleys, is thus at a maximum. When the motorvehicle now has to be towed in this condition, serious problems canarise:

[0004] The drive wheels of the vehicle guide torque into thetransmission which is in the pressureless state. Said torque istransmitted to the drive wheel via the output shaft and the belt-drivemeans. In the pressureless state, since a minimum friction radiusadjusts on the input side and a maximum friction radius accordingly onthe output side, the input shaft has to accelerate very intensively. Inthe pressureless state, since the compression force of the cone pulleyson the primary side is minimal, a skidding of the belt-drive means uponthe primary pulleys can occur under said intensive acceleration wherebysaid primary pulleys are seriously damaged.

[0005] Due to the skidding, it can also occur that one clutch connectedwith the input shaft is intensively accelerated even when the vehiclemotor is idle. When a certain limit rotational speed is exceeded, anundesired engaging of the clutch can occur in the clutch arrangement dueto the rotational pressure then appearing.

[0006] Due to the trailing torque then prevailing, an overheating of theclutch and the damage thereof can result.

[0007] The problem is solved by a transmission which, together with thefeatures of the preamble, also has the features of the characteristicpart of the main claim.

[0008] According to the invention, the aforementioned problems aresolved by the fact that the load piston of the adjusting pulley of theinput shaft is loaded with spring tension via a spring device whereby,when the transmission is in the pressureless state, the adjusting pulleyis axially displaced in a direction toward the fixed pulley. The appliedpressure of the cone pulleys thereby increases upon the belt-drivemeans, preventing a skidding of the belt-drive means during towing. Inaddition, the primary pulleys move in the axial direction on top of eachother whereby the friction radius of the belt-drive means upon theprimary pulleys increases and the ratio diminishes. The spring tensionacts here only during the pressureless state of the transmission and,therefore, does not affect regulation of the variator during normaldriving operation. Besides, the number of operating strokes of thespring device minimizes and this acts positively upon the service lifethereof.

[0009] A first advantageous embodiment is implemented by at least oneplate spring mounted outside a first pressure chamber of a contactpressure device of the adjusting pulley of the primary side. Said platespring braces a first load piston with a shaft-tied cylinder and isadvantageously centered on a cylindrical part of the first load piston.Thereby the load piston, in the pressureless state, is axially pressedaway from the cylinder to the fixed pulley. The adjusting pulley thusmoves and increases the contact pressure upon the belt-drive means and,likewise, the friction radius of the belt-drive means.

[0010] One other advantageous embodiment contains a second plate springwhich, the same as the first plate spring, is centered on a cylindricalpart of the load piston. Said second plate spring braces itself betweenthe first plate spring and the cylinder. The first plate spring bracesitself between the second plate spring and the outer side of the loadpiston. The spring tension upon the load piston and also the springtravel are increased without great expense by the second plate spring.

[0011] In both embodiments, in further advantageous arrangements, thefirst, the same as the second plate springs, can be centered on theirradially inner side.

[0012] A circlip can be installed, as needed, so as to make a stokelimitation of the plate spring possible by simple means. With saidstroke limitation, the travel of the plate spring can be adapted todifferent designs of variators. Besides, the service life of the platespring can be substantially improved by the limitation of the stroke.

[0013] In the pressurized state of the transmission, the plate spring ispermanently kept at its maximum engaged state. Thus, it does not affectthe regulation of the variator during normal driving operation. Besides,the number of operating strokes of the spring is thereby reduced, whichhas a positive effect on the service life thereof.

[0014] Advantageous embodiments are to be understood from the figures,which show:

[0015]FIG. 1 show parts essential to the invention of a continuouslyvariable belt-drive transmission;

[0016]FIG. 2 shows a first spring arrangement; and

[0017]FIG. 3 shows a second spring arrangement.

[0018]FIG. 1 shows a view of the parts essential to the invention of acontinuous belt-drive transmission. One first input shaft 1 carries theprimary pulleys 2, 3 which are designed with tapered friction surfaces25, 26. One of the pulleys is designed as fixed pulley 2 fixedlysupported on the shaft and one as an axially movable adjusting pulley 3.Between the primary pulleys 2, 3 lies a belt-drive means 4 which, duringthe driving operation, transmits torque from the input shaft 1 to theoutput shaft 5. Upon the output shaft 5 are supported secondary pulleys6, 7 which are also designed with tapered friction surfaces. A firstclutch 8 is fixedly connected with the input shaft 1. The contactpressure devices 9, 10, next to the adjusting pulleys, arediagrammatically shown.

[0019]FIG. 2 shows a detailed view of an advantageous arrangement. Uponthe input shaft 1 sits an axially movable cone pulley, the so-calledadjusting pulley 3. In addition, a shaft-tied cylinder 11 is fastenedupon the input shaft 1. On the side of the cylinder 12 facing theadjusting pulley 2, a plate spring 13 is pressed against a first loadpiston 14. The plate spring 13 is centered at its radially outer side 15by a cylindrical part of a load piston 16 and, in addition, it presseswith a radially outer edge 15 against the shaft-tied cylinder 11. Theplate spring 13 presses with its radially inner edge 17 against the loadpiston 14. A circlip 18 is inserted in the cylindrical part of the loadpiston 16. Said circlip 18 serves as stroke limit for the plate spring13. The spring stroke can thereby be adapted by simple means to thetotal ratio of the transmission. The first load piston 14 with theadjusting pulley 3 form a first pressure chamber 19. The cylinder 11with a second load piston 20 and the input shaft 1 form a secondpressure chamber 21.

[0020] In one other advantageous embodiment, the plate spring 13 canalso be centered at its radially inner side 17.

[0021] In another advantageous embodiment, the plate spring 13 can alsobe inversely built by pressing with its radially inner side 17 againstthe cylinder 11 and with the radially outer side against the load piston14. At the same time, the pressure surface of the shaft-tied cylinder 12is advantageously elongated inwardly in a radial direction.

[0022]FIG. 3 shows another advantageous embodiment of the invention. Asecond plate spring 22, the same as the first plate spring 13, iscentered in a cylindrical part of a load piston 16. Said second platespring 22 braces itself between the first plate spring 13 and theshaft-tied cylinder 11, pressing against the first plate spring 13 withits radially outer edge 30 and against the shaft-tied cylinder 11 withits radially inner edge 29. The first plate spring 13 braces itselfbetween the second plate spring 22 and the load piston 14. By itsradially inner edge 17 it presses against the load piston 14 and by itsradially outer edge 15 against the second plate spring 22. By saidsecond plate spring 22, the spring tension upon the load piston 14 andalso the spring stroke can be increased without great expense.

[0023] In this embodiment the first plate spring 13 can be inverselyinstalled in which case its radially inner side 17 presses against asecond plate spring 22 and its radially outer side 15 presses againstthe load piston 14. For this purpose the second plate spring 22 isadvantageously applied to the radially inner side 29 of the shaft-tiedcylinder 11.

[0024] In this embodiment, the plate spring 13 can also be centered, inanother design, on its radially inner side.

[0025] Reference Numerals  1 input shaft 17 radially inner end of theplate  2 fixed pulley spring  3 adjusting pulley 18 circlip  4belt-drive means 19 pressure chamber  5 output shaft 20 load piston  6fixed pulley 21 pressure chamber  7 adjusting pulley 22 plate spring  8clutch 23 outer edge  9 contact pressure device 24 inner edge 10 contactpressure device 25 tapered face 11 cylinder 26 tapered face 12 pressureface of the cylinder 27 tapered face 13 plate spring 28 tapered face 14load piston 29 inner edge 15 radially outer end of the plate spring 30outer edge 16 cylindrical part of the load piston

1. Continuously variable belt-drive transmission, specially for motorvehicles, in which a belt-drive means (4) runs between two pairs of conepulleys (2, 3, 6, 7) which are mounted on an input shaft (1) and anoutput shaft (5) and of said pulleys one is designed as fixed pulley (2,6) and the other as axially displaceable adjusting pulley (3, 7) andeach is constructed with tapered friction faces (25 to 28) andrespectively have available one contact pressure device (9, 10) foraxial displacement of said adjusting pulleys (3, 7), characterized inthat in pressureless state of the transmission, said contact pressuredevice (9) ensures, by means of a plate spring (13), a basic contactpressure upon said adjusting pulley (3) of said input shaft (1), saidplate spring (13) being located outside a first pressure chamber (19)and in engaged state braces a first load piston (14) with a shaft-tiedcylinder (11).
 2. Continuously variable belt-drive transmissionaccording to claim 1, characterized in that by its radially outer edge(15), said plate spring (13) loads said shaft-tied cylinder (11) with anaxial force counter to the direction to said fixed pulley (2) an dinaddition is centered on its outer edge (15) in a cylindrical part of aload piston (16).
 3. Continuously variable belt-drive transmissionaccording to claim 1, characterized in that on its radially inner edge(17), said plate spring (13) loads a load piston (14) with an axialforce in direction to said adjusting pulley (3).
 4. Continuouslyvariable belt-drive transmission according to any one of the precedingclaims, characterized in that the stroke of said plate spring (13) islimited by a circlip (18) so that in the pressureless state the ratio ofthe transmission remains higher than iV=1.
 5. Continuously variablebelt-drive transmission according to any one of the preceding claims,characterized in that in the pressurized state of the transmission, saidplate spring (13) is permanently kept at its maximum engaged state. 6.Continuously variable belt-drive transmission according to any one ofthe preceding claims, characterized in that a second plate spring (13)is used which is centered in the cylindrical part of said load piston(16) and on its radially outer edge (30) introduces a force upon saidplate spring (1) axially in direction of said load piston (14) and onits radially inner edge (29) introduces the counterforce toward saidshaft-tied cylinder (11).
 7. Continuously variable belt-drivetransmission according to any one of the preceding claims, characterizedin that said contact pressure device (9) of said adjusting pulley (3)has at least one second pressure chamber (21).